1. Field of the Invention
The present invention is directed to a golf ball and, more particularly, to a golf ball having an improved dimple pattern.
2. Description of the Related Art
Soon after the introduction of the smooth surfaced gutta percha golf ball in the mid nineteenth century, players observed that the balls traveled further as they got older and more gouged up. The players then began to roughen the surface of new golf balls with a hammer to increase flight distance. Manufacturers soon caught on and began molding non-smooth outer surfaces on golf balls, and eventually began to manufacture golf balls having dimples formed in the outer surface. Conventional dimples are depressions that act to reduce drag and increase lift. These dimples are formed where a dimple wall slopes away from the outer surface of the ball, forming the depression.
One method of packing dimples on a golf ball divides the surface of the golf ball into eight spherical triangles corresponding to the faces of an octahedron, which is a solid bounded by eight triangular plane faces. Dimples are then positioned within each of the surface divisions according to a placement scheme. The surface divisions may be further divided and the resulting subdivisions packed with dimples. Octahedron-based dimple patterns generally cover approximately 60-75% of the golf ball surface with dimples. U.S. Pat. Nos. 5,415,410 and 5,957,786 disclose octahedron-based dimple patterns.
Another dimple packing method divides the surface of the golf ball into 20 spherical triangles corresponding to the faces of an icosahedron, which is a polyhedron having triangular plane faces. Dimples are then positioned within each of the surface divisions according to a placement scheme. The surface divisions may be further divided and the resulting subdivisions packed with dimples. Because most icosahedron-based dimple patterns incorporate a high degree of hexagonal packing, they typically achieve more than 75% dimple coverage. U.S. Pat. Nos. 4,560,168 and 5,957,786 disclose icosahedron-based dimple patterns.
The dimples on a golf ball are important in reducing drag and increasing lift. Drag is the air resistance that acts on the golf ball in the direction opposite the ball's flight direction. As the ball travels through the air, the air that surrounds the ball has different velocities and, thus, different pressures. The air exerts maximum pressure at a stagnation point on the front of the ball. The air then flows around the surface of the ball with an increased velocity and reduced pressure. At some separation point, the air separates from the surface of the ball and generates a large turbulent flow area behind the ball. This flow area, which is called the wake, has low pressure. The difference between the high pressure in front of the ball and the low pressure behind the ball acts to slow the ball down. This is the primary source of drag for golf balls.
The dimples on the golf ball cause a thin boundary layer of air adjacent the outer surface of the ball to flow in a turbulent manner. Thus, the thin boundary layer is called a turbulent boundary layer. The turbulence energizes the boundary layer and helps move the separation point further backward, so that the layer stays attached further along the outer surface of the ball. As a result, there is a reduction in the area of the wake, an increase in the pressure behind the ball, and a substantial reduction in drag.
Lift is an upward force on the ball that is created by a difference in pressure between the top of the ball and the bottom of the ball. This difference in pressure is created by a warp in the airflow that results from the ball's backspin. Due to the backspin, the top of the ball moves with the airflow, which delays the air separation point to a location further backward. Conversely, the bottom of the ball moves against the airflow, which moves the separation point forward. This asymmetrical separation creates an arch in the flow pattern that requires the air that flows over the top of the ball to move faster than the air that flows along the bottom of the ball. As a result, the air above the ball is at a lower pressure than the air below the ball. This pressure difference results in the overall force, called lift, which is exerted upwardly on the ball. For additional discussion regarding golf ball aerodynamics, see copending patent application Ser. Nos. 09/989,191 entitled “Golf Ball Dimples with a Catenary Curve Profile,” filed on Nov. 21, 2001 and Ser. No. 09/418,003 entitled “Phyllotaxis-Based Dimple Patterns,” filed on Oct. 14, 1999, now U.S. Pat. No. 6,338,684.
Almost every golf ball manufacturer researches dimple patterns in order to increase the distance traveled by a golf ball. A high degree of dimple coverage is beneficial to flight distance, but only if the dimples are of a reasonable size. Dimple coverage gained by filling spaces with tiny dimples is not very effective, since tiny dimples are not good turbulence generators. Most balls today still have many large spaces between dimples or have filled in these spaces with very small dimples that do not create enough turbulence at average golf ball velocities.
The United States Golf Association (USGA) promulgates rules, one of which is directed to the symmetry of a golf ball. The USGA symmetry requirement dictates that a golf ball must be designed and manufactured to perform in general as if it were spherically symmetrical. Most dimple patterns tend to generate different flight characteristics based upon the orientation of the ball. For example, most icosahedron-based patterns have a tendency to fly slightly lower and longer in the poles-horizontal position (where the poles are oriented horizontally across the target line) than in the pole-over-pole, or poles-vertical, position. This is partially due to the manufacturing process; since most golf ball dimples are formed using a two-piece mold, the two pieces being mated at a parting line (i.e., the equator of the ball), most golf balls have at least one great circle that corresponds to the parting line of the molds and upon which no dimples are formed. In addition, most icosahedron-based patterns have more densely packed dimples near the pole than near the equator. Since the relative lack of dimples along the equator of the ball affects the aerodynamic performance of the ball, other areas of the ball must be modified in order to comply with the USGA symmetry rule.
One solution to the asymmetrical problem is to balance the parting line with additional great circles about the surface of the golf ball upon which no dimples are formed. These are known as “false parting lines.” Two such parting lines are typically used on an octahedron-based layout, bringing the total number of parting lines on the ball to three. One of the drawbacks of such patterns is that many dimples placed within the pattern will follow parallel latitudinal paths resulting in aligned rows of dimples, which can provide poor flight characteristics. (See U.S. Pat. No. 4,960,281 describing dimple non-alignment). Another drawback is that the multiple great circles reduce the percentage of the golf ball surface that can be filled with dimples.
Another way to overcome the asymmetry caused by the parting line is to alter the dimples around the poles. However, this raises the trajectory and shortens the distance of the poles-horizontal orientation to match those of the pole-over-pole orientation, lowering the overall aerodynamic performance of the ball.
Thus, what is needed is an improved dimple pattern for golf balls that provides high dimple coverage while simultaneously providing symmetrical flight characteristics.